In hydraulic machines with a closed circuit for the hydraulic fluid, like e.g. hydrostatic drives or propel systems, in particular in hydrostatic machines like hydrostatic pumps and hydrostatic motors the hydraulic fluid used for the generation of hydraulic forces and hydraulic performance suffers of elevated temperatures during operation of the machine or apparatus, respectively. Additionally, during operation, movable parts of the hydrostatic apparatus, as e.g. bearings, shaft bearings, guiding shoes, etc. need to be lubricated for its proper function as well as for being prevented of overheating. This lubrication and prevention of overheating is typically performed by a loop-flushing-system which creates, in addition to unavoidable leakages of valves, rotating and movable parts in the system, an intended fluid leakage from the low pressure side of the hydrostatic apparatus to a low pressure area of the hydraulic apparatus, e.g. to a housing of the apparatus or to a tank. Commonly, this fluid leakage in hydrostatic systems is filled up again by a charge pump pumping fresh and “cold” oil from the housing/tank into the hydraulic circuit at the low pressure side.
An implemented loop-flushing-system in a hydraulic apparatus controls the loop-flushing flow according to both the pressure level at the low pressure side and the pressure delta between the high and low pressure side in the hydrostatic circuit. The hydraulic fluid of the flushing system passes the movable parts, e.g. the bearings, for lubricating and cooling them. This deviated loop-flushing fluid flow is usually collected in the housing and/or in a tank for being conveyed further to a hydraulic fluid cooler before sucked again by the charge pump. The charge pump pressurizes the cooled hydraulic fluid and pumps it back into the low pressure side of the hydraulic circuit according to the pressure levels and operational conditions of the hydraulic apparatus.
The highest fluid temperature in a hydrostatic machine or system usually occurs in a hydraulic driving mechanism, like a rotational kit of a hydraulic machine. That is why the loop-flushing-system is frequently integrated in a hydraulic motor of a hydraulically driven system and flushes the fluid into the motor case, which is often connected to a tank by a drain line. Common loop flushing systems comprise a shuttle valve to select the low pressure side by using the low pressure as well as the high pressure for switching the shuttle valve into the adequate position. In its open position the shuttle valve opens a fluid path at the low pressure side to branch off hydraulic fluid from the low pressure line. Commonly, both end faces of a shuttle-valve spool are thereby pressurized either by low pressure or high pressure, respectively. If a predefined delta pressure between the low pressure side and the high pressure side is present the shuttle valve spool is shifted into one of its open positions and fluid connection to the area of low pressure could be enabled in cooperation with a flushing valve. If the pressure in a connection line between the shuttle valve and the flushing valve reaches a given threshold pressure the flushing valve opens as well and hydraulic fluid from the low pressure side of the hydraulic propel system could be flushed for lubricating and cooling of (moveable) parts of the hydraulic apparatus accordingly. After being collected in the housing or in a tank the used flushing fluid is passed then, for instance, to a cooler before being introduced again into the low pressure side of the hydraulic apparatus by the charge pump.
Such commonly known loop-flushing-systems are descript, e.g. in US 2014/0150880 A1. Here, a control valve is arranged between the shuttle valve and the flushing valve for overriding the normal function of the flushing circuit during certain machine operational events where flushing can cause undesirable performance issues. U.S. Pat. No. 6,430,923 B1 shows another known flushing circuit, in which the actuation of the shuttle valve spool is controlled by a microcontroller. A further flushing valve for closed hydraulic circuits of a hydraulic apparatus is shown in EP 2 314 897 A2. In this system the shuttle valve alone fulfills the selection function for detecting the high pressure side and the low pressure side as well as the function for flushing hydraulic fluid from the low pressure side to a tank. Hereby, flushing from the low pressure side is always enabled as long as the pressure in the low pressure line is higher than the tank pressure. Furthermore, this system allows a constant non-adjustable flow from the low pressure side to tank.
The disadvantages of all these flushing systems known from the state of the art are either that a permanent flushing takes place (EP 2 314 897 A2) or the loop-flushing systems can create pressure oscillations which could be carried forward to the low pressure side of the hydraulic circuit, at least producing an undesired, unusual and/or inexplicable noise. This e.g. occurs if the pressure at the low pressure side of the hydraulic propel system is slightly higher than the shifting pressure of the shuttle valve. In this state the shuttle valve is positioned in its opening position allowing hydraulic fluid connection between the low pressure side and the flushing valve. If the pressure in the connection line before the flushing valve is higher than the pressure needed to open the flushing valve the flushing valve opens and drains hydraulic fluid into the low pressure area. By opening the flushing valve the pressure in the connection line before the flushing valve drops down. If the pressure drops under the pressure level necessary for maintaining the flushing valve open, the flushing valve closes again. Subsequently, the pressure in the connection line before the flushing-valve spool increases as the pressure in the low pressure side of hydraulic propel system is higher than the opening pressure of the shuttle valve, such that the flushing valve opens again if the pressure force on the flushing valve spool is high enough. If the pressure excess in the low pressure line is less than the pressure drop occurring when opening the flushing valve, the flushing valve closes again. This periodical opening and closing of the flushing-valve occurs as long as the pressure at the low pressure side of the hydraulic device does not exceeds the opening pressure of the flushing valve by an amount higher than the pressure drop caused by the opening of the flushing valve. These oscillations of opening and closing the flushing-valve generate/cause a noise insinuating damages in the hydraulic system. Further, these oscillations may also hinder the effective and/or optimal cooling of the hydraulic fluid and the movable components of the hydraulic system as no controlled constant flush flow can be established. These oscillations are further carried back into the low pressure line probably causing further disturbances.